When you set up to the ball, your trail wrist is at a 90-degree angle to your swing arc. As you swing back to waist high, your trail wrist is still at a 90-degree angle to the swing arc. As your arm swings to the top of your swing, your trail arm externally rotates in your shoulder socket. That rotation makes the clubface look as though it has opened up. As you swing down, the arm internally rotates back so that at waist high, the wrist is back to a 90-degree angle to the arc. It then stays in that same relationship until waist high on the follow-through. At that point, your lead arm continues to rotate externally.